Prosthetic limbs as presently used are designed for optimum function and esthetic appearance. The art of fabricating prosthetic limbs with natural appearance is quite advanced, but the function of the limbs has been dependent upon secondary muscle control rather than primary muscle control except in kineplastic procedures. The concept of a percutaneous skeletal extension being used as a permanent functional weight-bearing protrusion or extension from the bone is not new but a major problem to the realization of such a permanent endoprosthetic device has been the attainment of a permanent intact skin-prosthetic interface.
The development of a protruding skeletal extension suitable for attaching a functional artificial limb has progressed through a number of design changes. Each change has been an attempt to solve identified and defined problems. A review of past mistakes and successes led to the establishment of the following criteria for future development of this type of device:
(1) The device must be a skeletal extension penetrating the skin in such a manner that the normal loads are transmitted directly to the skeletal system and not through thick layers of intervening soft tissues. PA1 (2) These loads must be distributed in such a manner so as not to damage the prosthesis, the bone to which it is attached, or any interfacial tissue ingrowth. PA1 (3) Both gross and microanatomical limitations must be kept in mind so that the device neither restricts the circulation nor otherwise impedes tissue healing. PA1 (4) In its final application, the skeletal extension must be a functional unit that permits freedom of motion and causes no pain. PA1 (5) The design should preferably permit minor adjustments to be made externally rather than require secondary operative procedures. PA1 (6) The device must have a surface suitable for tissue adhesion and/or ingrowth both at the bone interface and at the skin interface. The skin interface must prohibit the development of a sinus tract and inhibit bacterial invasion. PA1 (7) All materials used in fabrication must be compatible with interfacing tissues, must become functional for the purpose intended, and must not cause adverse systemic reactions. PA1 (8) The total end product must be readily sterilizable, using routine hospital procedures, prior to implantation. PA1 (9) The device should be designed to permit easy application under standard operating room conditions. PA1 (10) Ultimately, the design should permit use of existing skeletal muscles to power external articulating mechanical joints. This of course, demands development of an artificial tendon that will provide a strong tenacious interface with the musculotendinous portion of the existing skeletal muscles, penetrate the skin without allowing any entrance for bacterial invasion, and transmit the muscle's power to the load in an efficient manner.
The integument or skin is the body's first line of defense against microbial invasion. In the presence of implanted foreign material, particularly a protruding skeletal extension attached directly to the bone, it becomes even more important to maintain the integrity of the skin. Once the bacterial barrier of the integument has been broken, infection occurs and leads to the rejection of the implanted foreign material.
Our experience with skin interfacing has been reported in several articles. Metals, plastics, and ceramics have been tried, utilizing a variety of surface topography, including solids, textiles and foams. Some investigators have found carbon a useful skin interfacing material. No material has yet been found that is considered to be ideal, although Dacron, i.e. polyethylene terephthalate, and nylon velour fabrics bonded to a solid surface so as to form impervious laminates have thus far offered the most suitable solution.
Another problem is bone interfacing. Certain types of porous ceramics and sintered metals allow new bone ingrowth into the porous structure of the material.* It is also possible to have adhesion of bone to a solid non-porous surface. However, bone interfacing in general and surface ingrowth in particular are much more difficult and often disturbed when presented with a dynamic load such as would be applied constantly to a functional endoprosthesis device. FNT * See, for example, U.S. Pat. Nos. 3,808,606 to Tronzo and 3,855,638 to Pilliar.
Primary muscle extension or a functional attachment under the control of the primary muscles is greatly desired in a prosthetic limb.
Artificial tendons are known, but they have generally been attached by means of sutures or the like. The simple use of sutures has consistently led to failure due to stresses concentrated on the sutures. Either the suture breaks or it tears through the tissue. However, some alternatives have been suggested as noted by the following patents.
U.S. Pat. No. 3,745,590 to Stubstad shows an articulating prosthesis for a body joint which requires unrestricted orbiting motion with a flexible ligamentous element attached. The ligamentous element is affixed to the prosthesis device and adapted to be tied or otherwise affixed to an adjacent tendon, ligament or bone. The combination prosthesis/ligament is contained entirely within the human body. No part extends beyond the skin.
The Treace U.S. Pat. No. 3,953,896 discloses a prosthetic ligament for replacing a natural ligament flexibly connecting two skeletal members together. U.S. Pat. No. 3,973,277 to Semple et al discloses a tendon prosthesis and means to attach a tendon, either natural or artificial, to bone.
Stoy et al, U.S. Pat. No. 3,987,497, discloses another tendon prosthesis, and a suitable material for the core of the artificial tendon which will give it satisfactory physical properties such as tensile strength and elasticity. The Treace U.S. Pat. No. 3,988,783 shows a prosthetic ligament for replacing one of the collateral ligaments of the knee joint. It includes a bridge member and connector elements at the ends of the bridge member to connect to the bones of the leg.
The Homsy U.S. Pat. No. 3,992,725 relates to implantable material and appliances and methods of stabilizing body implants. One particular human implantation use of the growth-promoting material is as a prosthetic tendon. The growth-promoting material is bonded to the ends of the artificial tendon so that the tendon can be attached to the muscle at one end and to the bone at the other end.
In our earlier attempts, we used nylon and Dacron velour bonded to artificial tendon, the velour serving to attach the tendon to the musculotendinous portion by providing a site for tissue ingrowth. When such technique proved unsatisfactory, an impervious layer was bonded to the back of the velour to improve performance.
To attach an artificial tendon to a musculotendinous portion of a skeletal muscle may not per se pose difficult problems but to form an interface that will maintain the union under the repeated stresses of dynamic loading is a major problem. Furthermore, to bring the tendon out through the skin for external loading presents substantial additional problems. As with the skeletal extension itself, the integrity of the integument must be maintained in a manner that prohibits bacterial invasion.
One form of skeletal extension utilized experimentally has been an intramedullary rod held in position by friction.* But problems arose because the intramedullary rod interrupted the bone's main circulatory supply via the nutrient artery within the medullary canal. Thus, this device proved unsatisfactory as a skeletal extension. FNT * A Permanently Attached Artificial Limb, Hall et al, Trans. Amer. Soc. Artif. Int. Organs, 1967, Vol. XIII, pp. 329-331.